A master cylinder includes a housing with a pair of pistons which are movable during a brake application to develop fluid pressure. The pair of pistons cooperate with the housing to substantially form a pair of pressure chambers communicating with respective brake circuits. One of the pressure chambers communicates with its brake circuit via a proportioning valve assembly which is pressure responsive to vary the communiction of fluid pressure to this brake circuit. Generally, the proportioning valve cooperates with the brake circuit associated with the rear wheels to limit pressure buildup therein, as the tilting motion of a vehicle during braking reduces traction to the rear wheels. Such a master cylinder is illustrated generally in U. S. pat. No. 4,078,384.
It is desirable to proportion the communication of fluid pressure to the rear wheels when the fluid pressure developed in the rear brake circuit is above a predetermined value. However, if the front brake circuit should fail for some reason, it is important that some device be provided to prohibit proportioning. This is necessary because the rear brake circuit is the only brake circuit braking the vehicle when the front brake circuit fails, and it is not desirable to reduce brake pressure to the only braking circuit.